This disclosure relates to a method for controlling gallium content in garnet scintillators during a manufacturing process. In particular, this disclosure relates to a method for controlling gallium content in gadolinum-aluminum-garnet scintillators during the manufacturing process.
Gadolinium aluminum gallium garnets (commonly known as GAGG) are promising candidates for use as a scintillator in time of flight (TOF) positron emission tomography (PET) because of its high density of 6.63 grams per cubic centimeter (g/cc), high light output of greater than 65,000 photons/MeV (million electron volts or mega electron volts), and relatively short decay time of 88 nanoseconds (ns)/91% and 258 ns/9%.
GAGG can be grown in the form of large crystal boules of up to 3 inches (about 7.5 centimeters) in diameter from oxides such as cerium dioxide (CeO2), gadolinium oxide (Gd2O3), gallium oxide (Ga2O3) and alumina (Al2O3) that have a purity of 99.99% or greater using Czochralski method. A boule is a single crystal ingot produced by synthetic means.
One of the drawbacks associated with the Czochralski method is that the high temperatures (exceeding 1300° C.) used in the production of the crystal boule result in the decomposition of the gallium oxide to Ga2O vapor according to the reaction:Ga2O3Ga2O+O2  (1)This is an equilibrium reaction and the presence of additional oxygen in the reaction chamber reduces the rate of decomposition of the gallium oxide. In other words, the presence of an increased amount of oxygen in the reaction chamber drives the reverse reaction towards the formation of the gallium oxide rather than towards the formation of the Ga2O vapor.
The production of the crystal boule is generally conducted in iridium crucibles that are very expensive because of the cost of iridium metal. The use of large amounts of oxygen in the reaction chamber causes the conversion of iridium metal to iridium oxide (which evaporates), which is undesirable because of the high cost associated with the loss of iridium metal.
In order to effect a compromise and to obtain the GAGG crystal boule without any loss or iridium metal, an off-stoichiometric mix of raw oxide materials having 3 weight percent (wt %) excess of the gallium oxide is used to perform the growth process in an atmosphere containing an excess of 2 volume percent oxygen. The presence of excess gallium oxide compensates for the loss due to evaporation of gallium at the elevated process temperatures.
However, this method has its drawbacks as well. Since the evaporation losses of gallium are practically difficult to control, the quality of GAGG crystals may vary significantly. Loss of stoichiometry may cause substantial non-uniformities in the scintillation characteristics of the GAGG crystals such as: variation in light output, uncontrollable scintillation decay time, and a high level of afterglow, all of which are undesirable.
There therefore remains a need for a method to produce crystal boules of the correct stoichiometry while at the same time reducing the losses of gallium oxide or of iridium metal.